part 1] THROUGH THE ANDES OF PEKU AND BOLIVIA. 47 



is indicated. A comparison of the faunal list given here with that 

 of the limestone of the Titicaca district l further hears out this 

 suggestion, for only one or two species are common to hoth. 



The basement-heds are seen to rest with a well-marked uncon- 

 formity on an older series of rocks, comprising olive-green sand- 

 stones, quartzites, and black shales with concretionary limestone- 

 bands. These beds are strongly folded, the dip in places being 

 nearly vertical, and, if we judge from the frequent discordance in 

 their strike, they appear to have been much faulted, as shown 

 in the accompanying sketch-map (fig. 5, p. 48). With the ex- 

 ception of countless worm-tracks and a single obscure specimen 

 of a lamellibranch shell, they proved to be unfossiliferous. Their 

 lithological character suggests that they are of Devonian age, but 

 it was not considered advisable to separate them definitefy from 

 the older Palaeozoic rocks with which they are continuous. 



The latter, comprising hard grits, greywackes, and dark slates, 

 when first met with have a more or less constant dip westwards, but 

 ilt the summit of the Quellosani Pass they are inclined in a north- 

 easterly direction, and between this point and Macusani they are 

 much folded. In general appearance these beds closelj r resemble 

 the ancient rocks of the Pongo Valley, on the eastern slopes of 

 the Bolivian Cordillera, which are regarded as being of Silurian or 

 even earlier age ; and they are grouped together here, tentatively, 

 as Lower Palaeozoic, for fossiliferous beds of similar character wei-e 

 met with farther east. 



A short distance beyond Macusani commences the descent of the 

 San G-aban river-gorge, and one immediately encounters a remark- 

 able suite of igneous rocks which differ totally in character from 

 those described above as forming the plutonic core of the Western 

 Cordillera. With these are associated wide tracts of schistose l-ocks 

 presenting the appearance of highly-altered sediments, while the 

 igneous rocks themselves locally show signs of intense crushing 

 with the production of gneissic structure. The district is one 

 that has evidently undergone profound dynamic metamorphism ; 

 but, as the Cordillera has been subjected to more than one period 

 of orogenic movement, it is difficult to say to which of these such 

 a result should be attributed. I can see no valid reason for sup- 

 posing these rocks to be of earlier date than the fossiliferous Lower 

 Palaeozoic rocks found still farther east, and I am tempted to regard 

 them as constituting an ancient resistant horst formed during the 

 period represented by the gap between Middle Devonian and Upper 

 Avonian deposits, and thus as having played an important part in 

 determining the structural features produced during subsequent 

 folding of the Cordillera. 



The evidence on which this assumption is based, however, is too 

 slight to be in any way conclusive, and it is given here merely in 

 the light of a suggestion for future work. 



At the head of the San Graban gorge, a short distance beyond 



1 Q. J. G. S. vol. hex (1914) p. 31. 



